U.S. Pat. No. 2,551,731 describes the preparation of polyesters and polyester-amides by reacting glycols with dicarboxylic acids of which at least one contains a heterocyclic ring, such as 2,5-furandicarboxylic acid (2,5-FDCA). Under melt polymerization conditions, using sodium- and magnesium methoxide as a catalyst, 2,5-FDCA and 2.5 equivalents of ethylene glycol or 2,5-FDCA dimethyl ester and 1.6 equivalents of ethylene glycol were reacted in a esterification step or transesterification step, respectively, at ambient pressure between 160 and 220° C., after which a polycondensation was carried out between 190 and 220° C. under a few mm Hg pressure. The polycondensation process took between about 5 to over 7 hours. The product had a reported melting point of 205-210° C. and readily yielded filaments from the melt.
In US 2009/0124763 polyesters are described, having a 2,5-furandicarboxylate moiety within the polymer backbone and having a degree of polymerization of 185 or more and 600 or less. These polymers are made in a three step process involving the esterification of the 2,5-FDCA or the transesterification of the diester thereof with a diol, and a second step involving polycondensation, followed by solid state polymerization as third step.
The first step is carried out at ambient pressure at a temperature within a range of 150 to 180° C., whereas the polycondensation step is carried out under vacuum at a temperature within a range of 180 to 230° C. The product is then purified by dissolving the same in hexafluoroisopropanol, re-precipitation and drying, followed by the third step, a solid state polymerization at a temperature in the range of from 140 to 180° C. For the preparation of poly(ethylene furandicarboxylate) the first two steps took over 11 hours.
In WO 2013/120989 a continuous process for the preparation of poly(ethylene furandicarboxylate) is described wherein 2,5-FDCA or a diester thereof is mixed with ethylene glycol at elevated temperature to give a paste or a homogeneous solution, the paste or solution is converted to an esterification product of 2,5-FDCA and ethylene glycol, the esterification product is polycondensed under reduced pressure, wherein the polycondensation is performed in two stages. According to an example the dimethyl ester of 2,5-FDCA was reacted with ethylene glycol in a molar ratio of 1:1.7. In this example the stages following the production of the esterified product took 5 hours. The polycondensation product can be subjected, if desired, to a solid stating polymerization.
These patent documents do not refer to any coloration of the resulting polyesters. However, it is known that 2,5-furandicarboxylate-containing polyesters suffer from coloration. They tend to become yellow to brown. Such coloration has for instance been mentioned in P. M. Heertjes et al., Delft Progr. Rep., Series A: 1 (1974) 59-63 and Y. Hachihama et al., Technol. Repts. Osaka Univ. 1958, 8, 475-480.
In WO 2010/077133 a process for preparing furandicarboxylate-containing polyesters is described wherein the diester of 2,5-FDCA is transesterified with a diol, and the ester composition thus obtained is subjected to polycondensation in the presence of different tin catalysts. The polycondensation is conducted for a period of up to 5 hours. The polycondensate may then be subjected to solid state polymerization. In an example the solid state polymerization was conducted for 60 hours. Although the polyester obtained had good transparency properties, the process takes very long. An improvement is described in WO 2013/062408, wherein the dimethyl ester of 2,5-FDCA is transesterified with ethylene glycol, or bis(2-hydroxyethyl)-2,5-furandicarboxylate is used as starting material. The transesterification product or this starting material is then subjected to polycondensation and after a drying/crystallization step the polycondensate is subjected to solid state polymerization. The polycondensation was shown to take three hours. In an example the solid state polymerization takes two days. Although the object of the process of WO2012/062408 was to prepare a polyester with a high molecular weight without suffering from discoloration, no information on the color is provided in the specification.
KR 20140003167 describes a polyester polymer with an allegedly acceptable transparency, which is manufactured by using a biomass-originated 2,5-furandicarboxylate ester compound with ethylene glycol. In comparative examples also 2,5-furandicarboxylic acid has been used. The molar ratio of 2,5-furandicarboxylate ester to ethylene glycol may be from 1:1.1 to 1:4. The ratio of 2,5-furandicarboxylic acid to ethylene glycol varies between 1:1.2 to 1:2. No solid state polymerization is mentioned.
WO 2014/204296 describes polyesters comprising dicarboxylic acid residues and diol residues, wherein the dicarboxylic acid comprises 2,4-furandicarboxylic acid. These polyesters are stated to be surprisingly colorless or slightly yellow. In a comparative experiment the preparation of poly-ethylene-2,5-dicarboxylate is shown, wherein 2,4-durandicarboxylic acid dimethyl ester is reacted with 1,2-ethanediol, synonymous to ethylene glycol, in a molar ratio of 1:2. A number average molecular weight, determined by gel permeation chromatography using HFIP (hexafluoroisopropanol) as solvent of about 19,000 is reported. No information on the end groups or on the color of this polyester has been provided.
The above documents show that the coloration of polyesters that comprise 2,5-furandicarboxylate units is a re-occurring problem. Any measure that would mitigate this problem would be welcomed. Moreover, the solid state polymerization of many known methods takes a very long time. There is therefore a desire to enhance the solid state polymerization rate.